1. Field of the Invention
The present invention relates to the use of urea sulfate or urea hydrochloride in processes for making paper and pulp. In particular, the present invention relates to the use of urea sulfate or urea hydrochloride in place of mineral acids such as sulfuric acid or hydrochloric acid, or in place of aluminum sulfate in papermaking and pulping processes.
2. Description of the Related Art
The making of pulp and/or paper from wood and other cellulosic materials involves a number of different process steps known in the art.
Initial stock preparation may be done by an in-house pulping process in an integrated paper mill, or by a separate pulping process. Pulping is the conversion of wood or other cellulosic plant material into a slurry of fibers, and may be done by mechanical processes (e.g., grinding, thermomechanical refining), chemical processes (e.g., kraft, bisulfite, soda, or nitric acid digestion), or a combination thereof. Mechanical pulping processes typically produce a pulp having a much higher content of lignin than chemical pulping processes do. However, even chemical pulping often does not remove sufficient lignin to yield bright, flexible paper. As the pulping process continues, the rate of cellulose dissolution increases to the point where it exceeds the rate of lignin dissolution. As a result, the pulping process must be stopped and more compounds that more selectively dissolve lignin must be added. This is termed “bleaching,” and results in a whiter, brighter paper. Bleaching typically involves contacting the pulp with an oxidizer, such as a chlorine compound, such as hypochlorite or chlorine dioxide, or with an oxygen compound, such as ozone, oxygen, peroxide, etc. The effectiveness of bleaching with hypochlorite is highly pH dependent.
Pulp is delivered to the papermaking process per se as an aqueous slurry in integrated mills, or as dried sheets or “laps” which must then be dissolved in water to form the slurry. The objective in preparing the slurry is to disperse the fibers in water with as little mechanical work as possible, in order to avoid undesired modification of the fiber properties.
Nevertheless, almost all pulps are subjected to certain controlled mechanical processing, often called “beating,” in order to improve the strength and other physical properties of the paper sheet, and to influence the behavior of the system during papermaking. For instance, the fibers are often mechanically manipulated to swell, cut, macerate, and fibrillate them, at least in part in order to make them more flexible, thereby increasing the amount of hydrogen bonding between the fibers, which increases the strength of the paper formed therefrom.
The beaten pulp is then typically modified by addition of fillers, such as mineral pigments or clays, to affect opacity, brightness, and/or other paper properties. Sizing, or the addition of materials such as rosin, to modify the ability of the paper to absorb liquids, such as water, is often also added at this stage. Dyes can also be added during stock preparation, in order to produce colored paper. Significant quantities of alum (aluminum sulfate) are often added in conjunction with the rosin and/or mineral fillers and/or dyes in order to increase the amount of these materials that remains on the paper sheet. Other additives, such as starches, gums, modified celluloses, urea-formaldehyde and/or melamine-formaldehyde polymers, and other resins can also be added at this stage.
Once the stock has been prepared and properly diluted, it is then furnished to the papermaking machine, which is typically of the cylindrical or of the Fourdrinier type. The Fourdrinier type machine is described herein, as it is more common, but those of skill in this art will recognize that the description can be easily adapted to other papermaking equipment.
The properly diluted stock, or “furnish,” is screened, spread to the width of the machine, and discharged into a headbox. In the headbox, the furnish is subjected to microturbulence to minimize flocculation, and the desired pressure head is obtained. This pressure head causes the furnish to flow at the proper velocity out of the headbox, through a slice, and onto the Fourdrinier wire, which is a long, continuous wire screen supported by a breast roller, a couch roller, and by foils and suction boxes between the rollers to improve drainage of water from the resulting web of wet fibers. The foils are wing-shaped devices that support the wire and induce a vacuum at the downstream nip, which helps to pull water from the web and through the wire. Suction boxes provide additional dewatering.
Many modern Fourdrinier papermaking machines have twin wire units in order to provide better control of forming and dewatering, and more stable high speed operation. These machines remove water from the web by pressure, rather than vacuum, with the web pressed between the two wire screens around a cylinder or supporting wires or foils. Modifications of this nature are often referred to as Inverform or Bel Bond processes.
The process equipment described above is typically categorized as “wet end” equipment. The paper web that leaves the wet end has a greatly decreased water content, which has been separated into a process stream known as “white water.” The white water contains fiber debris, separated filler and sizing, etc., and after filtering and solids recovery, is reused for dilution of pulp stock into furnish. Environmental concerns dictate that white water reuse be maximized, however, this can lead to increasing concentration of dissolved chemicals as the number of recycles increases.
The web sheet leaving the wet end must be further dewatered, and this is typically accomplished by pressing and drying, using methods known to those of skill in the art (such as press rolls and continuous felts, steam cylinders, etc.). The dried paper is often then further treated, or “converted,” by subjecting it to operations such as pigment coating, embossing, impregnating, saturating, laminating, etc.
The processes involved in making paper and pulp require a number of different steps that require modification of the pH of solutions, or the use of mineral acids or aluminum sulfate.
For example, the pulping step converts wood or other cellulosic material into a high-cellulosic pulp by digesting all or a portion of the lignin in the material. A number of different pulping processes exist, the most common of which are the so-called kraft process, wherein pulping is carried out at alkaline pH, and the various bisulfite processes, which can be carried out at a variety of pH levels.
The kraft process involves cooking wood chips or other cellulosic plant material in a cooking solution containing caustic soda and sodium sulfide. Although the kraft process itself is not amenable to significant pH adjustment during the cooking process, pulps having a high α-cellulose content often must be “prehydrolyzed” with acid in order to degrade hemicelluloses. This must occur prior to cooking because the alkaline conditions of the kraft process will stabilize the hemicelluloses to further alkaline attack. Prehydrolysis, which is typically carried out with sulfuric acid or hydrochloric acid (about 0.25% to about 0.5%), also dissolves bonds between hemicelluloses, celluloses, and lignins, without damaging the cellulose. Pulps prepared according to this process are typically used as “dissolving pulps” in the manufacture of viscose and cellulose derivative products, rather than in making paper.
Kraft process pulps used in paper manufacture are typically washed, and the washing liquor is recycled back to various points in the pulping process. This recycling results in an increasing concentration of pulping chemicals and byproducts. This may include, for example, alkaline materials that cause the pH of the washing liquid to rise to unacceptably high levels.
Bisulfite pulping processes can be carried out at a variety of pH levels, and lend themselves to pH adjustment, since the reactivity of lignin is essentially determined by the pH of the pulping liquor, and since the active pulping chemical is different at different pH levels. For example, at pH levels of 1–2 (acid (bi)sulfite pulping), sulfur dioxide is the predominant active pulping agent. At pH levels of 3–5 (bisulfite pulping), bisulfite and hydrogen ion are the active pulping agents. At pH levels of 5–7 (neutral bisulfite pulping), bisulfite and sulfite ions are the active pulping agents. At pH levels of 9–13.5 (alkaline bisulfite pulping), sulfite and hydroxyl ions are the active pulping species. The respective pH levels of these pulping processes can be adjusted downward as necessary by adding thereto acids or materials that will form acids in aqueous solution, such as sulfur dioxide, sulfuric acid, hydrochloric acid, etc.
In addition, pH adjustment is important in the so-called multistage bisulfite pulping processes, where pulping is carried in two or more stages at different pH levels, and the pH is adjusted up or down between stages. For example, in the so-called Stora process, the first pulping stage is at a pH of 6–8 and the second stage is at a pH of 1–2. Again, this pH adjustment can be accomplished by adding sulfur dioxide, sulfuric acid, nitric acid, or other mineral acid to the pulping mixture.
Another pulping process that is occasionally used for pulps with high market value is the nitric acid pulping process, where the high cellulose pulps from, e.g., beech wood, is digested by impregnating chips with nitric acid and cooking, followed by alkali extraction with caustic. The cooking time can be significantly reduced by adding sulfuric acid or aluminum sulfate to the pulping mixture.
Other pulping processes include the acetic acid process (which uses H2SO4) or the acetosolv or phenol processes (which use HCl).
In addition, pH adjustment and solution neutralization is also required in the stock preparation, pulp bleaching, dyeing, and papermaking processes. In addition to the use of mineral acids to adjust pH in these processes, papermaking alum, or aluminum sulfate hydrate (Al2(SO4)3.18H2O) is often used. Alum is often added in large quantities to help precipitate rosin sizing onto the paper. This typically occurs at a pH of about 4.5 to 5.5, and may also involve the presence of sulfuric acid. Alum also functions as a retention aid (in combination with other retention aids), in that it helps to keep filler particles in the paper, as well as helping to control sheet formation by helping to flocculate fiber and improve drainage of water. Alum is used to increase wet and dry strength of the paper by facilitating adsorption of bridging polymers onto the paper by neutralizing negative charges on water-soluble particulate impurities, and by catalyzing the insolubilization of urea-formaldehyde or melamine-formaldehyde resins. Alum is used to control pitch formation on processing equipment by flocculating the pitch and keeping it on the paper. The flocculating ability of alum is also used to precipitate dyes, in particular acid dyes, onto the paper. Finally, alum is used in a “save-all” treatment of white water to remove finely divided solids therefrom.
Mineral acids and alum can also be used to control pH in papermaking and pulping effluent streams. The papermaking and pulping industries have come under increasing pressure to decrease the amount of process water used and the amount of pollutants discharged with spent process water. This involves increasing use of recycled water, which over time accumulates increasing amounts of solid particulates, and in alkaline processes, increasing alkalinity. Alum can be used to precipitate or flocculate the particulates from these effluent and recycle streams, and alum and mineral acids can be added to neutralize residual alkalinity before the streams are recycled or discharged into the environment.
Thus, despite the overall movement to “alkaline” papers for printing of books, mineral acids and alum are extensively used in the paper and pulping industries. It is estimated that about 5% of the total annual U.S. production of virgin sulfuric acid was used in the pulp and paper industry (approximately 4 billion lbs/yr). The consumption of alum by the pulp and paper industry is similarly quite large. A significant amount of hydrochloric acid is also consumed by the paper and pulp industry. These compounds, however, present a number of disadvantages. Sulfuric acid is highly corrosive to process and storage equipment, requires significant and expensive precautions to handle, and requires that significant and expensive reporting requirements be met with regard to transport and storage. Aluminum sulfate is similarly regarded as corrosive by the U.S. Department of Transportation. Both sulfuric acid and aluminum sulfate can form insoluble sulfates, such as calcium sulfate, when they come into contact with calcium hydroxide, which is extensively used in the paper and pulp industry. The build up of insoluble salts on processing equipment can lead to costly shutdowns for cleaning and maintenance. Hydrochloric acid is similarly corrosive, and gives off objectionable fumes as well.
Accordingly, there exists a need in the art for a material that is safe and effective at adjusting pH in pulp and paper processing streams, that is less corrosive to process equipment, that is easy and safe to handle, store and transport, that is not subject to government reporting requirements. In addition, there is a need for a pH adjusting material that does not give off objectionable fumes, and that does not form large quantities of insoluble salt products, such as calcium sulfate, which can deposit on process equipment.